The 1st Central County Ground outfit will be in Division Three of the Division Stage, alongside Worcestershire, Middlesex, Leicestershire, Kent, and Derbyshire, after finishing bottom of Group 3.
Iain Salisbury's men will begin the Division Stage away at Worcestershire on Monday, August 30 (11am start).
Sussex then welcome Middlesex to Hove on Monday, September 6 (10.30am), before making the trip to Leicestershire on Sunday, September 12 (10.30am).
Sussex conclude their 2021 County Championship campaign with a home game against Derbyshire on Tuesday, September 21 (10.30am).
Prize money at the end of the County Championships will be awarded based on finishing positions across all three divisions.